As the material requirements for gas turbine engines continually increase, considerable emphasis has been placed on improved alloys characterized by relatively low densities and high strength at elevated temperatures. Titanium-based alloy systems have been developed as a result of this requirement, with notable success occurring with titanium intermetallic systems based on the titanium aluminide TiAl (gamma). Gamma titanium aluminide alloys typically contain aluminum in amounts between about 46 to about 52 atomic percent, and are generally characterized as being relatively light weight, yet exhibiting high temperature strength, stiffness and burn resistance. As such, considerable effort has been directed toward evaluating these gamma titanium aluminide alloys for aerospace structural components which have been typically formed from nickel or titanium alloys.
Generally, gamma titanium aluminide alloys (also referred to as gamma alloys) exhibit relatively low ductility and low fracture toughness at room temperature, making these alloys difficult to process. In addition, unless properly alloyed, gamma alloys do not exhibit desired high oxidation resistance due to their tendency to form titanium dioxide (TiO.sub.2) rather than aluminum oxide (Al.sub.2 O.sub.3) at high temperatures. For example, the oxidation limit for a gamma alloy is often significantly less than its creep limit. Accordingly, a common objective with the use of titanium aluminide alloys is to achieve a good balance between mechanical properties at both room temperature and elevated temperatures, and environmental characteristics such as oxidation resistance.
U.S. Pat. No. 4,879,092 to Huang, assigned to the same assignee of the present patent application, teaches a gamma titanium aluminide alloy whose composition is nominally, in atomic percent, 48 percent aluminum, 2 percent chromium and 2 percent niobium, with the balance being titanium and incidental impurities (48Al--2Cr--2Nb). This alloy exhibits strength and environmental resistance comparable to nickel alloy Alloy 718 at the upper temperature limit of Alloy 718. Furthermore, the 48Al--2Cr--2Nb alloy exhibits about fifty percent greater specific stiffness than conventional titanium and nickel alloys. As such, the alloy taught by Huang meets the requirements of many structural components for gas turbine applications.
The alloy taught by Huang is directed primarily toward wrought processing. As is well known in the art, wrought gamma alloys inherently have microstructural features that differ significantly from gamma alloys that have been processed by casting. Such differences directly affect such properties as strength, ductility, creep resistance and fracture toughness. While the 48Al--2Cr--2Nb alloy taught by Huang has been identified as having desirable properties in wrought form, this alloy does not fully exploit the unique properties of gamma alloys for cast applications.
Research directed toward TiAlCrNbTa gamma alloys has been reported by Austin and Kelly in "Development and Implementation Status of Cast Gamma Titanium Aluminide", Structural Intermetallics, The Minerals, Metals & Materials Society (1993). While this research generally indicated that chromium, niobium and tantalum content had an effect on the ductility of a gamma alloy, nothing was reported as to their effects on creep strength, which is a key concern for components subjected to stresses while operating at high temperatures. Notably, little is known or taught in the prior art concerning creep effects of chromium, niobium and tantalum on gamma alloys.
Accordingly, it would be desirable to provide a gamma titanium aluminide alloy whose chemistry is optimized for cast applications, and is characterized by elevated temperature strength and environmental resistance, enabling cast components to operate at temperatures higher than that possible with prior art gamma alloys. It would be particularly desirable if the creep strength of a gamma titanium aluminide alloy were optimized in order to permit castings formed from such an alloy to be used as gas turbine engine structural components that are subjected to temperatures of 650.degree. C. and more, yet are required to maintain their dimensional tolerances.